Fuel cells using solid polymer electrolytes were proposed in 1950s and have been developed ever since for the purpose of supplying spacecraft with energy.
Beyond the generation of power for spacecraft, interest in fuel cells has progressed. Particularly, the automobile industry has interest in them for two reasons. The first reason is related to the increased concern for avoiding pollution caused by internal combustion engines. In fact, it is very difficult to prevent all discharges caused by internal combustion engine, such as nitrogen oxides, hydrocarbons caused by incomplete combustion and acidic compounds by means of all the improvements that one can expect through better control of combustion. The second reason, for the long term, is to research motors that use fuel other than fossil fuel which is known not to last forever.
Any fuel cell system based on hydrogen or methanol can respond to the concerns mentioned above. The sources of fuel, hydrogen and methanol, are potentially inexhaustible and their electrochemical combustion only produce water.
The schematic assembly of a fuel cell that produces electrical energy and water at the same time is represented in FIG. 1.
The ion exchange type of membrane formed from a solid polymer electrolyte (1) is used to separate the anode compartment (4) where oxidation of the fuel such as hydrogen or methanol occurs according to the equation:2H2->4H++4e− (hydrogen fuel cell)CH6OH+H2O->CO2+6H++6e− (direct methanol fuel cell)
from the cathode compartment (5) where the oxidant such as oxygen is reduced according to the equation:O2+4H++4e−->2H2O (hydrogen fuel cell)3/2O2+6H++6e−->CO2+2H2O (direct methanol fuel cell)
with production of water (7) while the anode and the cathode are connected through external circuits(6).
The anode (8) and the cathode (9) are essentially constituted by a porous support, for example made of carbon, on which particles of a noble metal such as platinum or ruthenium are deposited.
The membrane-electrode assembly (MEA) is a very thin assembly with a thickness of the order of a millimeter. Each electrode is supplied from the rear with the gases using a fluted plate with serpentine flow path. One very important point is to properly maintain the membrane in an optimum hydrated state so as to ensure maximum proton conductivity.
The membrane has a double role. On one hand, it acts as a proton conducting polymer permitting the transfer of hydrated proton (H3O+) from the anode to the cathode. On the other hand, it effectively separates oxygen, hydrogen and/or methanol as a buffer. Therefore, the polymer constituting the membrane must therefore fulfill a certain number of conditions relating to its mechanical, physico-chemical and electrical properties.
First, the polymer must be able to be prepared into thin membranes with a thickness of 50 and 100 micrometers, which are dense and without defects. Its mechanical properties, especially tensile stress, modulus and flexibility must make it compatible with preparation condition of the MEA which is to be clamped between metal frames. Further, the properties must be conserved simultaneously from a dry to a hydrated state.
In addition, the polymer must have good thermal stability to hydrolysis and exhibit good resistance to reduction and oxidation up to 100° C. In particular, in order to be used for direct methanol fuel cell, the polymer electrolyte membrane must not allow methanol to pass through the membrane from anode to cathode.
Finally, the polymer must have high ionic conductivity, which is provided by acidic groups such as phosphoric acid groups and sulfonic groups linked to the polymer chain. Therefore, these polymers will generally be specified by their equivalent mass, that is to say, acid equivalent per the weight of polymer in grams (Ion Exchange Capacity).
Since 1950, numerous families of polymers or sulfonated polycondensates have been tested as electrolyte membranes for fuel cell. At present the relationships among chemical structure, film morphology and performance are established.
At first, sulfonated phenolic type resins prepared by sulfonation of polycondensed products such as phenol-formaldehyde resins were used.
The membranes prepared with these products are advantageous in terms of low cost, but they do not have sufficient stability to hydrogen at 50–60° C. for applications of long duration.
Next one turned towards sulfonated polystyrene derivatives which have greater stability in comparison with those of the sulfonated phenolic resins, but the sulfonated polystyrene derivatives are disadvantageous in that they cannot be used at a higher temperature than 50–60° C.
At the present time, the best results are obtained with copolymers that have linear perfluorinated main chain in back bone and graft side chains with sulfonic acid groups.
These copolymers are commercially available under the trademark Nafion from the Du Pont Company or ACIPLEX-S from Asahi Chemical Company. Others are experimental products, such as the membrane named “XUS” by the DOW Company.
Such polymers containing perfluorinated sulfonic acid groups, which have been the subject of numerous developments, conserve their properties for several thousands of hours between 80 and 100° C.
The polymers of the Nafion type can be obtained by co-polymerization of two fluorinated monomers one of which carries the sulfonic acid groups. Other routes for obtaining perfluorinated membranes have been explored in documents by G. G. Scherer: Chimia, 48 (1994), p. 127–137; and by T. Monose et al., U.S. Pat. No. 4,605,685. It involves the grafting of styrene or fluorinated styrene monomers onto previous sulfonated fluorinated polymers. These membranes have properties close to those of fluorinated co-polymers.
However, such Nafion type polymers may be limitedly applied in the manufacture of direct methanol fuel cell because methanol transfer from anode to cathode can easily occur even when that the concentration of methanol is very low, resulting in poor performance.
In addition, U.S. Pat. No. 6,245,881 shows various sulfonated polyimides prepared by copolymerization with diamine monomers having sulfonic acid groups. The publication reports that these sulfonated polyimides have excellent thermal stability and resistance to reduction, as well as high ion exchange capacity up to 2.5 meq/g.
However, there exist limited kinds of diamine monomers having sulfonic acid groups. In addition, the solubility and reactivity of those monomers are so poor that they cannot be easily resolved in most solvents except m-cresol, and the degree of polymerization is too low to form an adequate film.
The solubility of the monomers may be improved by substituting the hydrogen ion of the —SO3H group by +1 metal ion such as Li+, Na+ and K+. These modified monomers become soluble in other solvents such as dimethylsulfoxide (DMSO). However, the polymers prepared from such monomers have poor solubility in most other solvents, and the metal substituted sulfonic acid group cannot be easily returned to its original form, SO3H in order to be used as a cation exchange membrane.
Further, because of the strong rigidity of polyimides whose backbone structures are basically composed of aromatic monomers, the introduction of SO3H groups into their main chains prevents the morphologies of the produced films from being uniform.
From the above, it can be known that the polymers for the manufacture of effective polymer electrolyte membranes must have high proton conductivity, excellent thermal and mechanical properties and low gas permeability. Also, their chemical structures must be able to prevent the leakage of fuel such as methanol.